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1 Tectono-magmatic, sedimentary and hydrothermal history of Arsinoes and 2 Pyrrhae Chaos, Mars
3 Erica Luzzi1, Angelo Pio Rossi1, Cristian Carli2 and Francesca Altieri2
4 5 1Jacobs University, Bremen, Germany 6 2Inaf-IAPS Tor Vergata, Rome, Italy 7 Corresponding author: Erica Luzzi ([email protected])
8 Key points
9
10 • We produced a morpho-stratigraphic map of Arsinoes and Pyrrhae Chaos, including the
11 volcanic grabens occurring throughout the study area;
12 • Spectral analyses of the light-toned deposits provide clues for sedimentary and
13 hydrothermal minerals; spectral analyses of the bedrock are indicative of basaltic
14 compositions;
15 • The observed volcano-tectonic surface features and the lack of evidences of any fluvial
16 activity suggest that magmatic processes might be primarily responsible for the collapse
17 of the chaotic terrain.
18
19 Abstract
20 Arsinoes and Pyrrhae Chaos are two adjacent chaotic terrains located east of Valles Marineris
21 and west of Arabia Terra, on Mars. In this work we produced a morpho-stratigraphic map of
22 the area, characterized by a volcanic bedrock disrupted into polygonal mesas and knobs
23 (Chaotic Terrain Unit) and two non-disrupted units. The latter present a spectral variation,
24 likely associated with hydrated minerals, and they are here interpreted as sedimentary units.
25 The reconstructed geological history of the area starts with the emplacement of the basaltic
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26 bedrock, followed by the collapse that caused the formation of the chaotic terrains. Since
27 evidences of volcano-tectonic activity are widespread across the area (e.g. fissure vents/graben,
28 radial and concentric systems of faults, y-shaped conjunctions, lava flows, pit chains), and an
29 intricate system of lava conduits is hypothesized for the occurrence of such features, we
30 propose the possibility that the whole collapse was caused primarily by volcano-tectonic
31 processes. In a late stage, after the end of the volcano-tectonic activity, a lacustrine/evaporitic
32 depositional environment could have set, with the deposition of the non-disrupted units. The
33 hydrated minerals found in the periphery of the Chaos could be the result of hydrothermal
34 alteration of the basaltic bedrock.
35 Keywords:Chaotic terrains; Mapping; Spectral analyses; Mars; Hydrothermal system.
36 Plain Language Summary
37 Chaotic terrains are peculiar features on Mars. They consist of broad regions characterized by a variable surface
38 disruption pattern of large polygonal blocks. Formation scenarios in the literature have always included a collapse,
39 possibly caused by a range of processes, all including water or hydrated compounds (magma-ice interactions,
40 melting of buried ice, groundwater pressure, etc.). In this work, we propose volcano-tectonic processes as
41 mechanism of formation for closed chaotic terrains. Additionally, our mineralogical analyses suggest that during
42 a late stage of the volcanic activity a hydrothermal system could have set. In such scenario hot water would have
43 risen from the subsurface through fractures created by the volcanic activity, evolving from eruptive to
44 hydrothermal. However, water would not have been directly involved in the initial collapse that formed the chaos.
45 1 Introduction
46 Arsinoes and Pyrrhae Chaos are two adjacent chaotic terrains, respectively centered at 7.8°S,
47 332°E and 10.3°S, 331.5°E (Fig. 1), a few tens of kilometres south of Aureum Chaos and a few
48 hundred kilometres SW of Aram Chaos, sharing with the latter many structural and
49 depositional characteristics. Several mechanisms of formation were proposed in literature to
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50 explain the nature of the putative collapse responsible for the disruption of the bedrock into
51 polygonal blocks that characterizes the chaotic terrains. The proposed scenarios include: i) a
52 major role played by groundwater and cryosphere, particularly linked to changes of pressure
53 within the aquifer that caused the disruption of the bedrock and subsequent water outflow
54 (Andrews-Hanna & Phillips, 2007; Carr, 1979; Harrison & Grimm, 2009; Rodriguez et al.,
55 2005), ii) the occurrence of a buried ice lake that after melting would have caused fracturing
56 and catastrophic outflow (Manker & Johnson, 1982; Roda et al., 2014; Zegers et al., 2010), iii)
57 catastrophic destabilization of buried clathrates (Hoffman, 2000; Kargel et al., 2007), and iv)
58 magma-cryosphere/groundwater interactions (Chapman & Tanaka, 2002; Head & Wilson,
59 2007; Leask et al., 2006; Meresse et al., 2008; Wilson & Head III, 2002). Given the complexity
60 of the current geologic setting of Arsinoes and Pyrrhae Chaos, possibly augmented by an
61 extended time of erosion and mantling, a possible interaction between the proposed processes
62 (or singular contributions at different times) must also be considered.
63 In the present work we performed the morpho-stratigraphic mapping of Arsinoes and Pyrrhae
64 Chaos and a spectral analysis of the deposits. We propose a possible sequence of events to
65 explain the occurrence of the collapsed bedrock involving magmatic processes followed by an
66 aqueous depositional environment.
67
68 1.2 Regional setting
69 The first comprehensive description of chaotic terrains was made by Sharp et al. (1971) using
70 Mariner 6 imagery, followed by Sharp (1973) based on Mariner 9 data and Schultz et al. (1982)
71 based on Viking data. In this early stage of research, the main features of Martian chaotic
72 terrains were already clear and these areas were defined as deeply collapsed terrains disrupted
73 into an irregular pattern of tilted mesas and knobs, in some cases associated with outflow 3
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74 channels. The Chaotic Terrain Unit was first defined by Schultz and Rogers (1982) and then
75 with a refinement by Glotch & Christensen (2005) into three subunits: Fractured Plains,
76 Knobby Terrain and High Thermal Inertia Chaotic Terrain. The Chaotic Terrain Unit is
77 interpreted as the basaltic bedrock (Christensen et al., 2000; Glotch & Christensen, 2005) and
78 occurs over a large area including several other chaotic terrains such as Aureum, Aram,
79 Hydratoes, Aurorae, Chryse and Hydaspis Chaos. The age of the Chaotic Terrain Unit
80 according to Tanaka et al., (2014) is Middle Noachian, with younger Hesperian ages in the
81 internal portions of collapsed areas. With the collection of new compositional data from
82 THEMIS (Thermal Emission Imaging System), TES (Thermal Emission Spectrometer),
83 OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) and CRISM
84 (Compact Reconnaissance Imaging Spectrometer for Mars), the investigations in literature
85 focused mainly on the sedimentary units lying on top of the basaltic bedrock. Several authors
86 analyzed the mineralogy of the layered sedimentary units occurring for example in Aram Chaos
87 (Catling & Moore, 2003; Christensen et al., 2001; Dobrea et al., 2008; Gendrin et al., 2005;
88 Glotch & Christensen, 2005; Lichtenberg et al., 2010; Liu et al., 2012; Massé et al., 2008;
89 Ormö et al., 2004), Aureum and Iani Chaos (Dobrea et al., 2008; Glotch & Rogers, 2007;
90 Sefton-Nash et al., 2012). Hematite deposits associated with monohydrated and polyhydrated
91 sulfates were spatially correlated with layered sedimentary units, separated by an unconformity
92 from the basaltic bedrock. The occurrence of hydrated sulfates and hematite allowed some
93 authors to assume that an aqueous and/or hydrothermal depositional environment must have
94 set after the collapse of the bedrock. The sedimentary layered deposits in Arsinoes Chaos were
95 not included in the previous studies, while in Pyrrhae Chaos the sedimentary deposits are not
96 observed at all. The lack of studies in this location emphasizes the need to expand our
97 knowledge of this area, providing a broader context on Martian chaotic terrains.
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98 2 Data and methods
99 2.1 Data, processing and tools
100 The imagery used to perform the geological mapping is provided by the CTX (Context Camera)
101 (Malin et al., 2007) and HiRISE (High Resolution Imaging Science Experiment) (McEwen et
102 al., 2007) instruments onboard the MRO (Mars Reconnaissance Orbiter). One HRSC (High
103 Resolution Stereo Camera, on board Mars Express spacecraft) image was also used to observe
104 in false colour the study area and one HRSC DEM (Digital Elevation Model) was downloaded
105 for the topographic contours in eastern Pyrrhae Chaos, since the area is not covered by CTX
106 stereo pairs. CTX imagery was used as a basemap; in particular we used a global blended CTX
107 mosaic provided by the Murray Lab (Dickson et al., 2018). HiRISE images were used to
108 observe in detail the stratigraphic contacts in certain areas: EDR products were processed and
109 tiled through the USGS software ISIS3 (Gaddis et al., 1997). The data processing was
110 supported by GNU Parallel (Tange, 2011). Variations in thermal inertia were investigated on
111 JMARS (Java Mission-planning and Analysis for Remote Sensing) (Christensen et al., 2009)
112 developed by ASU's Mars Space Flight Facility. JMARS is a Geographic Information System
113 (GIS) where different layers can be loaded on a global basemap: it was used to visualize the
114 THEMIS (Thermal Emission Imaging System) Derived Global Thermal Inertia Mosaic.
115 DEMs from CTX were computed using ASP (Ames Stereo Pipeline) developed by NASA
116 (Beyer et al., 2018; Moratto et al., 2010): the resulting products are bundle-adjusted to the
117 global topography (MOLA - Mars Orbiter Laser Altimeter). A list of the used images is
118 provided in Table 1.
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119 The imagery was then imported into ESRI Arcgis for the geological mapping. The bedding
120 attitudes were measured using the beta version of LayerTools (Kneissl et al., 2011), kindly
121 provided by Dr. Thomas Kneissl.
122 The CRISM (Murchie et al., 2007) cubes (S detector - short wavelength channel, and L detector
123 - long wavelength detector) with full resolution (FRT) available in the study area (also listed
124 in Table 1) were processed by means of the software ENVI (with the CAT extension) where
125 atmospheric corrections and projection were applied, allowing the subsequent visualization and
126 analysis of the spectra.
127 2.2 Mapping
128 2.2.1 Scale
129 The geological map of the area was digitized on ArcGIS at the CTX resolution (~5 m/px), but
130 the chosen output scale is 1:3.000.000.
131 Only craters with a diameter larger than 2 km were mapped.
132 2.2.2 Polygonal features – Morpho-stratigraphic units
133 The units observed in the study area are five and were mapped as polygons. The units include
134 the three subunits of the Chaotic Terrain unit (Fractured Plains, Knobby Terrain and High
135 Thermal Inertia Chaotic Terrain) and the non-disrupted units (Light-toned Layered Unit and
136 Cap unit). Furthermore, the inner part of the post-collapse craters was mapped as an additional
137 unit: Post Collapse Craters. Following the USGS guideline (2005), we chose warm colours for
138 the Chaotic Terrain subunits given their volcanic nature. On the other hand, we chose to use
139 cold colours for the non-disrupted units since they were interpreted as sedimentary deposits
140 and green for the craters so that they could be clearly distinguished from the bedrock.
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141 2.2.3 Linear features – Structural features and contacts
142 The mapped linear features include the crater rims, the contacts between different units and the
143 structural features. The fractures affecting the bedrock and the elongated graben-like
144 depressions were included in a single category called grabens. This choice is mainly due to the
145 coalescence of the structures and the difficulty in distinguishing them. The second group of
146 structural features is represented by the wrinkle ridges. The contacts were divided into certain
147 contacts (continuous lines) and approximate/inferred contacts (dotted lines), for those cases
148 where the mantling covers the contact (e.g. between Cap Unit and Light-toned Layered Unit)
149 or if the change from one unit to another is transitional (e.g. between Knobby Terrain and High
150 Thermal Inertia Chaotic Terrain).
151 2.2.4 Surface features – Pit chains and pitted areas
152 Pit chains and diffuse pits are widespread throughout the study area. In several parts the high
153 density of pits did not allow to map individual pit chains, leading to the necessity to introduce
154 a polygonal feature to indicate these areas (Pitted areas).
155 2.2.5 Symbols
156 The symbology used for crater rims, grabens, pit chains, wrinkle ridges, certain and
157 approximate contacts was chosen in agreement with the standards recommended by USGS and
158 FGDC for planetary mapping (FGDC, 2006). For pitted areas we introduced a new symbol.
159 3 Results
160 3.1 Morphological observations and stratigraphy
161 The stratigraphic relationships indicate that the oldest unit observed in Arsinoes and Pyrrhae
162 Chaos is the Chaotic Terrain Unit, composed of three subunits: Fractured Plains (characterized 7
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163 by angular flat-topped blocks), the Knobby Terrain (displaying rounded mounds) and the High
164 Thermal Inertia Chaotic Terrain (heavily eroded and characterised by the higher thermal
165 inertia). The Fractured Plains occur predominantly along the rim of the Chaos, while the
166 Knobby Terrain acts as a transition towards the inner parts of the Chaos. The Knobby Terrain
167 subunit is the most extensive material representing the Chaotic Terrain Unit in this area. The
168 Knobby Terrain is in contact with the sharp angular mesas of the Fractured Plains (Fig. 2) and
169 the small mounds of the High Thermal Inertia Chaotic Terrain. The latter occurs only in a small
170 area in the NE part of Arsinoes Chaos, in contact with the Knobby Terrain, and in a crater
171 within Pyrrhae Chaos, in contact with the Fractured Plains. The rounded mounds characterizing
172 the Knobby Terrain are located at lower elevations compared to the mesas of the Fractured
173 Plains (Fig. 2). The scarps bounding the mesas of the Fractured Plains show a depth up to 1 km
174 and the flat-topped blocks appear as irregular polygonal bodies. The mesas of the Fractured
175 Plains subunit show a layering implying multiple depositional events, nevertheless it is difficult
176 to follow the layers in lateral continuity on the adjacent mesas due to the different erosion and
177 therefore we could not constrain with precision this bedding.
178 Stratigraphically above the Chaotic Terrain Unit, non-disrupted and layered deposits lie
179 unconformably. In Arsinoes Chaos, the non-disrupted deposits overlying the Chaotic Terrain
180 Unit displays two different morphologies and attitudes. Furthermore, they occur at different
181 elevations showing their stratigraphic relationship. Therefore, they were considered as two
182 distinct units, the Light-toned Layered Unit (LLU) and the Cap Unit (Cap) (Fig. 3). At lower
183 elevations, the light-toned deposits are characterized by planar bedding and scalloped surfaces;
184 their morphology resembles the ILDs (Interior Layered Deposits) described by Sowe et al.
185 (2007), Le Deit et al. (2008) and Schmidt et al. (2018). The average attitude of the light-toned
186 layered deposits is ~ S80E, 20°. The LLU outflanks the mounds of the Knobby Terrain (Fig.
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187 4), assuming often a lobate shape and wrapping around the knobs. The overlying sedimentary
188 deposits (Cap Unit) are separated from the LLU by an unconformity: the attitude of the
189 youngest deposits (informally called Cap Unit due to the fact that these deposits “seal” the
190 succession) is ~ N70W, 3°. The Cap Unit does not seem to be layered or if it is, the bedding is
191 massive and the outcrop visible today represents only one thick bed with a plateau-like
192 morphology. The occurrence of these two non-disrupted units only in some areas of the Chaos
193 leads to the question of whether the current extent represents approximately the original extent
194 or the erosion obliterated a large portion of the deposits that were originally covering the entire
195 Chaos.
196 In Pyrrhae Chaos the non-disrupted sedimentary units are not present, although the two
197 adjacent Chaos show a similar depth (in certain areas of Pyrrhae Chaos the depth is even higher
198 than Arsinoes Chaos).
199
200 3.2 Structural observations
201 In Arsinoes and Pyrrhae Chaos, as well as in other chaotic terrains, the main structural feature
202 is represented by the deep fractures bounding the polygonal blocks of the Fractured Plains. The
203 fractures are open and show no displacement. Erosion and degradation might have enhanced
204 the opening of these fractures after their formation. The minimum depth of the fractures can be
205 assumed by the height of the mesa from the floor, and it is on the order of few hundred meters,
206 up to 1 km. Additionally, the floor of the chaotic terrain accommodates the non-disrupted units,
207 thus preventing the determination of the real depth of the blocks. The orientation of fractures
208 bounding the blocks follows two trends that show variations throughout the chaos. The
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209 polygonal geometries are due to the irregularly orthogonal disposition of these two trends (Fig.
210 5a-b).
211 A large number of elongated grabens were mapped (Fig. 5a-b). These confined structures, that
212 may resemble channels at a first glance, are often in coalescence with the orthogonal fractures
213 of the polygonal mesas (Fig. 5c). In addition, grabens are also in coalescence with pit chains
214 and/or occur in areas heavily pitted (Fig. 5d). Considering graben-like depressions and
215 fractures as belonging to the same group of structures, two patterns can be distinguished based
216 on their orientation: one group of structures seems to follow the rim of the Chaos, showing
217 therefore a concentric pattern with respect to the rim of the basin; another group shows instead
218 a radial pattern that was likely radiating from the center of the basin outwards but that is now
219 only visible in proximity of the Chaos rim, due to dust cover and younger overlying units
220 occurring within the basin. When these two sets cross each other, polygonal blocks are defined.
221 The elongated graben-like depressions have a linear or slightly sinuous morphology and do not
222 show any braided system nor meanders. The depth range is between 100 and 400 meters (up to
223 1000 if we consider those bounding the Chaos), while the length can reach up to 40 km.
224 Moreover, several elongated graben-like depressions display y-shaped bifurcations.
225 Although the non-disrupted deposits do not have complete lateral continuity, no major faults
226 affecting them were detected, not even at the HiRISE resolution. However, the lack of
227 continuity of these deposits due to erosion means we cannot exclude the existence of younger
228 faults affecting the layered deposits.
229 Compressive structures such as wrinkle ridges were also observed ~30 km SE from Arsinoes
230 Chaos (Fig. 6), providing an important clue on the existence of a compressional regional stress
231 prior the formation of the widespread grabens (east to Arsinoes Chaos a wrinkle ridge is cut by
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232 a graben). The wrinkle ridges show a typical orientation ~N-S and are characterized by
233 sinuous/arcuate morphologies.
234 The last important structural observation concerns two major pre-collapse craters that were
235 incorporated in the collapse, one in the north-eastern Arsinoes Chaos and one in the south-
236 eastern Pyrrhae Chaos (depicted in Fig. 7). The craters are both filled by material affected by
237 the grabens. The infilling has a maximum observable thickness of 1500 m in the crater located
238 at NE Arsinoes Chaos, while the thickness in SE Pyrrhae Chaos reaches only 1000 m. The
239 appearance of the infilling deposits seems to not differ from the surrounding basaltic bedrock,
240 suggesting that the impacts predate not only the collapse, but also the last volcanic resurfacing
241 events. The rims of the craters are partially degraded (in both craters, the southern rim is less
242 degraded than the northern rim) and remnants of the ejecta are only visible in the southeastern
243 crater. An exposed lava flow occurs in the crater at SE Pyrrhae (Fig. 8a). The lava flow is
244 darker and less eroded than the surrounding bedrock, less covered by dunes and dust (Fig. 8B),
245 and it is overlying all the volcanic deposits, suggesting that it may represent a late volcanic
246 resurfacing event. This last resurfacing event is still predating the collapse as evidenced in Fig.
247 8C, where both the lava flow and the underlying rest of the bedrock are cut by a graben.
248 3.3 Thermal Inertia
249 The surface temperature of a given area depends on the properties of the exposed materials,
250 but it is also affected by external factors such as dust covering and atmospheric pressure.
251 Diurnal changes in temperature can be detected and described through the thermal inertia, a
252 bulk property of materials defined by the relationship between thermal conductivity, density,
253 and specific heat of the considered material. These properties are different for each material
254 and some of the factors influencing the thermal behaviour are for example grain size,
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255 cementation, packaging of the grains and degree of exposure. Therefore, surfaces with different
256 thermal inertia are most likely indicative of a change in composition and/or physical properties.
257 A material with high thermal inertia is able to gather the heat and conduct it beneath the surface
258 during the day, while during the night the stored heat is released through the surface (Mellon
259 et al., 2000). In this way the surface appears cold during the day and warmer during the night.
260 High thermal inertia is typical of consolidated/lithified materials, such as an exposed bedrock,
261 lava flows, indurated and compact rocks; on the other hand unconsolidated sands and dust show
262 a low thermal inertia (Fergason et al., 2006). On Mars, thermal information is provided by TES
263 (Thermal Emission Spectrometer, see e.g. Jakosky et al., 2000; Christensen et al., 2001), on
264 board Mars Global Surveyor, and THEMIS (Thermal Emission Imaging System, see e.g.
265 Christensen et al., 2004), on board 2001 Mars Odyssey. We investigated the thermal inertia
266 visualising the THEMIS-Derived Global Thermal Inertia Mosaic on JMars.
267 As anticipated in the previous sections, two areas with higher thermal inertia were detected in
268 the study area. The first area occurs in Arsinoes Chaos and corresponds to the most eroded part
269 of the Chaotic Terrain Unit that was identified as the High Thermal Inertia Chaotic Terrain
270 subunit. The differentiation from the Knobby Terrain is transitional and delineating a sharp
271 boundary was not trivial, but the high thermal inertia seems to coincide with the most eroded
272 and peaked knobs. The second area is located in Pyrrhae Chaos and corresponds to the lava
273 flow previously described (Fig. 9), located in proximity of a set of grabens. In this case the
274 contrast is sharp and well-defined and the higher thermal inertia coincides perfectly with the
275 lava flow that differently from the surrounding basaltic bedrock does not present neither
276 mantling nor regolith (except for small dunes within the pits on the surface of the lava flow).
277
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278 3.4 CRISM spectral analyses
279 In the nearby Aram Chaos, Glotch & Christensen (2005), detected mixtures of sulfates and
280 phyllosilicates (associated with plagioclases and pyroxenes) in different percentage in all the
281 non-disrupted units (Cap Unit and layered units), using the TES and THEMIS datasets. Part of
282 the layered deposits was interpreted by the authors as hematite-bearing, while within other
283 layered deposits the iron oxide was not found. Moreover, Lichtenberg et al. (2010) provided a
284 stratigraphic and mineralogical characterization of the hydrated sulfates occurring in Aram
285 Chaos, based on CRISM data. The authors identified two sedimentary units: the oldest
286 consisting of monohydrated sulfates intercalated with ferric hydroxy-sulfate or nanophase
287 ferric oxides and the youngest bearing polyhydrated sulfates and crystalline hematite.
288 Monohydrated sulfates were detected by Lichtenberg et al. (2010) observing minor absorptions
289 at 2.1 and 2.4 μm; ferric hydroxy-sulfate was interpreted through absorptions at 2.238 μm,
290 associated with minor absorptions at 1.49, 1.82, and 2.38 μm; polyhydrated sulfates were
291 inferred from the absorptions at 1.9 and 2.4 μm within the youngest unit unconformably lying
292 on the oldest with monohydrated sulfates. Polyhydrated sulfates in association with crystalline
293 gray hematite have been found also by Dobrea et al. (2008) in Aram, Aureum and Iani Chaos:
294 using the TES and OMEGA datasets, the authors were able to point out a correlation between
295 all these chaotic terrains East of Valles Marineris. Sowe et al. (2012) analysed CRISM spectra
296 from Aureum Chaos, and also in this case the authors were able to identify within the light-
297 toned layered units hydroxylated, monohydrated, polyhydrated sulfates. The hydroxylated
298 sulfates were interpreted by the authors based on the absorptions at 2.23 μm (related to the
299 occurrence of OH), 1.42–1.45 μm and its weak 1.93 and 2.4 μm bands. Monohydrated sulfates
300 (kieserite) were detected through absorptions at 2.12 μm and a major absorption at 2.4 μm,
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301 while polyhydrated sulfates were diagnosed based on absorptions at 1.42–1.44 and at 1.92–
302 1.93 μm.
303 Considering these evidences from the nearby Aram and Aureum Chaos and given the
304 morphologic analogies, we investigate if similar mineralogies can be detected also in Arsinoes
305 Chaos.
306 Only one full resolution CRISM cube is available within the inner region of Arsinoes Chaos
307 (frt00008233_07_if164). This cube was analysed in order to characterize the non-disrupted
308 units overlying the basaltic bedrock. A first distinction of spectrally different terrains was made
309 based on the RGB images (Fig. 10) with combined summary products (Viviano-Beck et al.,
310 2014). First, we examined the minerals revealed by previous works on chaotic terrains. In Fig.
311 10C the summary parameter for chlorides shows a possible occurrence of hydrated minerals
312 (in yellow). The putative hydrated minerals coincide in extent with the morphologically
313 identified Light-toned Layered Unit. The RGB composites derived from the parameters that
314 are specifically used to identify hydrated minerals (such as sulfates and phyllosilicates) did not
315 provide a clear distinction of a mineralogical variation. Moreover, the difficulty in finding a
316 spectrum with clear evidences of hydrated minerals is attributed to the detection limit and to
317 the noise affecting the data. Therefore, summary products suggest a mineralogical variation
318 and likely bearing hydrated minerals, but the spectra do not allow the detection of specific
319 minerals. The interpretation of these deposits as sedimentary can only be supported by
320 analogies with the adjacent chaotic terrains and by the morpho-stratigraphic observations.
321 Other hydrated minerals (CRISM cube frt000196b0_07) were found in the northeastern
322 periphery of Arsinoes Chaos (Fig. 11), within a deposit in correspondence of some of the
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323 collapse features previously described. Slightly north of this area we performed also analyses
324 on the bedrock (CRISM cube frt00023790_07 and frt000196b0_07).
325 The hydrated minerals detected in the CRISM cube frt000196b0_07 are concentrated in the
326 central area of the cube, precisely coinciding in extent with an exhumed deposit slightly
327 different in albedo from the surrounding materials. As shown in Fig. 12A, a mineralogical
328 variation is already distinguishable from the infrared false color image where the hydrated
329 minerals appear to be pale green. This first clue is therefore additionally supported by the RGB
330 composites in Fig. 12B and 12C. In 12B the RGB composite PFM is shown for the detection
331 of Fe and Mg in the crystalline structure of the hydrated phyllosilicates, in particular for
332 significant band depths at 2.3 μm; as a result, Fe-Mg hydrated phyllosilicates are displayed as
333 cyan, allowing to appreciate the mineralogical variation at a first glance. For the ratioed
334 spectrum the numerator is an average of some of the cyan pixels shown in Fig. 12B having
335 both band depth at 1.9 and 2.3 micron well above the detection limit, while the denominator is
336 an average of pixels with relatively flat spectra picked from the same column of their respective
337 numerators. The resulting ratioed spectrum is reported in Fig. 12, where a comparison with
338 spectra from the CRISM resampled library is provided. Several phyllosilicates sharing similar
339 absorptions (indicative of Fe, Mg-OH) were plotted in order to compare even the weakest
340 absorption to understand the corresponding mineral that may occur in the deposit. We
341 interpreted the ratioed spectrum as smectite, since the ratioed spectrum and the smectite’s
342 spectrum share the same absorptions, including the absorption at 2.28 μm that is instead shifted
343 toward > 2.3 μm in the other plotted phyllosilicates, due to a bigger concentration of Fe instead
344 of Mg (Clark et al., 1990).
345 On the same CRISM cube frt000196b0_07 analyses of the bedrock revealed the occurrence of
346 mafic minerals, in particular pyroxenes with low Ca content. A first identification of the 15
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347 pyroxenes was made by analyzing the summary product LCPINDEX2 (low-Ca pyroxenes)
348 (Fig. 13). A significant spectrum for the analysis of the bedrock results from the CRISM cube
349 frt00023790_07, where broad absorptions at 1 μm and 2 μm confirm the occurrence of
350 pyroxenes (Fig. 14) (Viviano-Beck et al., 2014). The asymmetry of the absorption at 1 μm
351 towards longer wavelength could also be associated with the occurrence of a second mafic
352 phase as olivine or an high-Ca pyroxene, with variations in breadth due to the amount of Fe
353 and Mg or Ca (Cloutis et al., 1986; King & Ridley, 1987). Pyroxenes and olivine are indicative
354 of basaltic compositions. This observation supports the basaltic nature of the bedrock of the
355 chaotic terrain (Chaotic Terrain Unit).
356 3.5 Morpho-stratigraphic map
357 The observations carried on and described in the previous section led to the creation of a
358 morpho-stratigraphic map of the area (Fig. 15).
359 The units were classified through photo-interpretation, taking into consideration: visible and
360 sharp contacts, transitional contacts, differences in texture, degree of erosion and mantling,
361 morphology, bedding when occurring, relative relationships between units, attitudes and
362 compositional information where available. The bedrock of the area is identified in the Chaotic
363 Terrain Unit, a basaltic unit subdivided into three subunits: the Chaotic Terrain Fractured Plains
364 (ChF on the map), the Knobby Chaotic Terrain (ChK) and the High Thermal Inertia Chaotic
365 Terrain (ChH). Because of the volcanic nature of the Chaotic Terrain Unit, the three subunits
366 were mapped with warm colors. An unconformity separates the bedrock from the overlying
367 Light-toned Layered Unit (LLU), while a second unconformity is interposed between the LLU
368 and the overlying Cap Unit (Cap) that seals the succession. Spectral data showed the possibility
369 that LLU is composed by hydrated minerals, but the detection limit and the low quality of the
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370 CRISM cube did not allow a specific classification of the hydrated minerals. On the other hand,
371 hydrated minerals classified as Fe- and Mg-phyllosilicates were found in a small deposit in the
372 northeastern periphery of Arsinoes Chaos, but the limited extent did not allow to represent it
373 on our map at this scale.
374 Craters in the area with a diameter larger than 2 km were mapped as Post-collapse crater.
375 As for the other features (pit chains, wrinkle ridges, grabens, and crater rims), they were
376 mapped using the standard symbols FGDC. We introduced a new symbol for pitted areas,
377 where the pits were highly coalescent and concentrated making it difficult to map the single
378 feature.
379 The produced map contributes to fill the gaps regarding Chaotic terrains, but most importantly
380 it highlights the need to expand the availability of maps in this complex area in order to
381 facilitate future works and discussions.
382 4 Discussion
383 4.1 Pre-collapse events and Floor-Fractured craters
384 The information that we gathered on the pre-collapse stratigraphy is limited to a few
385 observations: i) the basaltic mesas of the Chaotic Terrain Unit present different layers,
386 suggesting multiple volcanic resurfacing events before the collapse; ii) the ancient impact
387 craters occurring in NE Arsinoes Chaos and SE Pyrrhae Chaos predate the collapse since they
388 are filled with materials affected by the collapse-related structures; iii) it is still unclear if the
389 three subunits of the Chaotic Terrain Unit represent just a different lateral erosional evolution
390 of the same material or if the different erosional patterns are due to a lateral mineralogical
391 variations as yet undetected. The craters in NE Arsinoes Chaos and SE Pyrrhae Chaos were
392 likely formed between the multiple eruptive events that emplaced the Chaotic Terrain Unit and 17
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393 afterwards they have been embedded into the collapse. The crater in NE Arsinoes Chaos,
394 presents a thicker infilling, a more eroded rim and no sign of ejecta: these could be clues of an
395 older age compared to the crater in SE Pyrrhae Chaos that has a thinner infilling and visible
396 traces of the eroded ejecta. We have to keep in mind that both of them are filled by fractured
397 materials; therefore, even though the southeastern crater may be younger, it still predates the
398 collapse and the end of the lava supply that formed the Chaotic Terrain Unit.
399 This particular type of craters affected by polygonal fractures were classified on Mars by
400 Bamberg et al. (2014) as FFCs (Floor-Fractured Craters). FFCs have been extensively studied
401 on the Moon: the origin of their fractures was attributed to two main processes. One of the
402 discussed processes is the viscous relaxation of the crater topography proposed by Hall et al.
403 (1981) but then proved wrong by Dombard & Gillis (2001); the other proposed process (the
404 most widely accepted) consists of a sill emplacement and consequent inflation (Jozwiak et al.,
405 2015; Jozwiak et al., 2012; Thorey & Michaut, 2014; Wichman & Schultz, 1996). Bamberg et
406 al. (2014) performed an accurate global classification of the martian FFCs, dividing them into
407 two groups: one group of FFCs occur in fluvial areas and were formed (or modified after the
408 formation) by fluvial activity. The second group (that includes our area of study) was attributed
409 to intrusive volcanism: the factors considered by the authors include absence of fluvial
410 morphologies and outflow channels, but also occurrence of volcanic pits, collapsed lava
411 conduits, lava sheets, and basaltic composition of the bedrock. We agree with this interpretation
412 and we propose that processes related to magmatic intrusion could explain on a large scale the
413 collapse of the chaotic terrain itself in absence of aqueous-related surface features.
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414 4.2 Bedrock collapse
415 The widespread occurrence of grabens leads to question their nature and the possibility that
416 they could be linked to the origin of the collapse. Following Scott & Wilson (2002), grabens
417 are interpreted here as volcanic graben. Similar structures within the Tharsis region, on
418 Olympus Mons and Ascraeus Mons, were interpreted as fissure vents by Mouginis-Mark &
419 Christensen (2005), but also as fluvial-related by Scott & Wilson (1999). Furthermore, grabens
420 similar to that occurring in Arsinoes and Pyrrhae Chaos were mapped and interpreted as vents
421 in Ascraeus Mons (Mars) by Pozzobon et al. (2015). Such vents would have erupted lavas
422 coming from a complex network of dykes, while collapsed pits would be associated with feeder
423 dykes. Dykes were invoked for the formation of grabens and pit chains also in Pavonis Mons
424 by Montési (2001), where the author describes the possible interactions between dykes and
425 volatiles and the resulting structures based on the degree of interaction. According to Montési
426 (2001), grabens are formed by deep dykes that do not reach the volatile-rich layer but stop at
427 the extensional stage; if the dyke is able to quickly reach the volatile-rich layer, their interaction
428 results into pits; coalescing pit chains would be formed by an intermediate interaction between
429 the dyke and the volatile-rich layer.
430 In first instance, we rule out the possibility that the grabens may have any connection with
431 fluvial systems: no braided nor meandering patterns were recognized, as well as no tributaries
432 nor typical fluvial depositional morphologies anywhere in the study area. Since outflow
433 channels or other fluvial features are lacking, a volcano-tectonic origin seems more likely,
434 considering the basaltic mineralogies across the area, the presence of y-shaped conjunctions
435 indicative of inflation and at least one evident lava flow spatially associated with the fissures
436 (Fig. 8A). Moreover, the grabens have a significant depth (up to 1000 m), that could be
437 explained by a constant flow (in case of water) and not by a periodic/stagnant river;
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438 nevertheless there are no evidences of such a developed hydrographic system, and the channels
439 terminate abruptly without fluvial deposits nor confluence into other channels. Additionally we
440 rule out that grabens were formed by groundwater sapping which, as evidenced by Lamb et al.
441 (2006) is unlikely to erode in short time a basaltic bedrock (and if it spanned over a long period
442 then we would expect more hydrated alteration of the basalts). Irwin et al. (2014) performed
443 remote sensing analyses on the valleys within the desert of Atacama and discussed the
444 possibility to infer if groundwater sapping caused the incision of the valleys on Mars or not.
445 The authors argue that theater-headed valleys on Mars should not be interpreted as due to
446 groundwater sapping by default because many other factors may influence the formation of the
447 headscarps, and the morphology of the valleys per sē cannot justify such interpretation.
448 According to Mège & Masson (1996), the grabens occurring in Valles Marineris were formed
449 during Hesperian as a passive rift system, but the authors do not exclude the possibility of an
450 interplay with magmatic processes related to the Tharsis region that occurred in a more recent
451 span of time. Tanaka & Golombek (1989) propose that tension fractures were responsible for
452 the formation of grabens on Mars, particularly in the area of Valles Marineris. These authors
453 argue that the orientations of grabens and pit chains suggest a structural control that cannot be
454 attributed to volcanic processes since no volcanic deposits occur in correspondence of these
455 structures. Nevertheless, more recent works (with access to younger and better datasets than in
456 1989) showed the occurrence of widespread basaltic mineralogies within the chaotic terrains
457 (Glotch & Christensen, 2005), small pitted cones interpreted as of volcanic nature (Meresse et
458 al., 2008), dykes within Valles Marineris (Flahaut et al., 2011; Mège & Gurgurewicz, 2016;
459 Okubo & Schultz, 2005) and lava flows draping the outflow channels (Leone, 2014).
460 Therefore, based on the mentioned references and on our observations, the grabens that we
461 described and interpreted as volcanic grabens (sensu Scott & Wilson, 2002) may be considered 20
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462 as the result of the collapse of lava conduits, or fissure vents, or due to inflation processes
463 originated from buried magma chambers and/or buried magma intrusions, or as due to the
464 ascent of dykes.
465 Another surface evidence for such collapses is represented by the extensive pits and pit chains,
466 that at a first glance may resemble small craters but lack in ejecta and raised rims. Pits and pit
467 chains were interpreted in literature as the surface collapse of buried lava conduits (Leone,
468 2014), as collapse due to magma stoping or as the result of explosive activity, likely due to
469 interactions between magma and H2O (Head & Wilson, 2002; Okubo & Martel, 2005; Wyrick,
470 2004), and as the result of the interaction between dykes and volatiles (Montési, 2001). An
471 additional interpretation based on a terrestrial analogue was proposed by Ferrill et al. (2011),
472 who observed pit chains in Iceland formed by the interplay of dilational faults, extension
473 fractures, and tectonic caves. The authors suggested a similar combination of mechanisms for
474 the formation of pit chains on Mars. In our study area, extensional faults with clear
475 displacements were not found, the polygonal blocks are different from the horst and graben
476 systems on Earth because arranged in radial and concentric patterns. The pits and pit chains are
477 often coalescent with volcanic grabens, suggesting perhaps extremely unstable conditions of
478 interconnected plumbing systems. Moreover, pit chains that are often coalescent and intersect
479 each other with different orientations suggest excluding a pure tectonic control.
480 An important topic that we want to mention is the origin of the outflow channels, absent in
481 Arsinoes and Pyrrhae Chaos but occurring in association with other chaotic terrains. Despite
482 the broad consensus of the scientific community on aqueous erosional processes carving the
483 outflow channels (Andrews-Hanna & Phillips, 2007; Baker, 2001; Carr, 1979; Harrison &
484 Grimm, 2008; Hoke et al., 2011; Leask et al., 2007; Meresse et al., 2008; Rodriguez et al.,
485 2005; Rodriguez et al., 2015), recently several authors proposed that the outflow channels 21
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486 might have been carved by lava flows (Jaeger et al., 2010; Leone, 2014; Leverington, 2011).
487 Leverington (2011) lists and discusses the evidence against an aqueous origin of outflow
488 channels: i) lack of fluvial deposits along the channels, together with deltas and sedimentary
489 shoreline features, while channels seem to be lava draped instead; ii) incompatibility between
490 the amount of required water and the global mineralogical observations of the unaltered olivine;
491 iii) paucity of analogues in the solar system of large-sized channels carved by aquifer outburst;
492 iv) the required permeability of the megaregolith might not be realistic. In contrast with the
493 aqueous origin of outflow channels the author highlights how a volcanic origin seems to be the
494 simplest explanation based on the evidences on the martian surface.
495 Koeppen & Hamilton (2008) performed a global analysis of the olivine end-member
496 mineralogies found on the martian surface: they postulated that the poor alteration of the olivine
497 could be interpreted as due to spatially inhomogeneous wet conditions in the early climate,
498 with very wide dry areas, or to the short-term availability of water on Mars.
499 Based on our overall observations in our area of study, we speculate that it would be possible
500 that the trigger of the outflow channels could have been magmatic bodies heating the
501 permafrost. The absence of outflow channels in our area of study could be explained by a lateral
502 variation of the permafrost thickness and/or possibly the local fracturing state of the uppermost
503 crust.
504 Reassessing the extent of the role played by water or lava in forming outflow channels is
505 beyond the aim of this work. The literature reviewed above follows two schools of thought, but
506 we believe that both water and magmatic processes contributed to the final geological setting
507 of chaotic terrains, one prevailing on the other and vice versa at different times.
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508 We observed hydrated mineralogies, especially in the periphery of Arsinoes Chaos, while in
509 the inner Chaos the detection of hydrated minerals is weaker and limited to the RGB
510 composites. The spectra there did not allow the identification of specific phases, probably
511 because their abundance is below the detection limit. Therefore, we confirm that water was
512 present but our interpretation, based on the stratigraphic relationships between the observed
513 geological units and structures, separate the major contributions of volcanic and aqueous
514 processes in two stages: during the first stage, magmatic processes were mainly responsible for
515 the collapse of the chaotic terrain, possibly interacting with volatiles and/or permafrost; in a
516 second stage (post-collapse) water played a major role (perhaps for a short time given the
517 limited extent of the hydrated deposits).
518 4.3 Thermal Inertia
519 The high thermal inertia of the lava flow occurring in Pyrrhae Chaos could be related to the
520 younger age of the lava flow than the surrounding bedrock. A high thermal inertia is normally
521 observed in lava flows, but the absence of high thermal inertia in the underlying basaltic
522 bedrock (except for the most eroded subunit, the High Thermal Inertia Chaotic Terrain, and for
523 the steep slopes of the mesas, recognizing that mantling debris on slopes may thermally obscure
524 to different extents) is due to the fact that the Chaotic Terrain unit is partially covered by
525 regolith and/or mantling. Our guess is that the basaltic bedrock underlying the lava flow could
526 be older than the overlying lava flow, and therefore more affected by mantling and weathering,
527 while the lava flow is still acting thermally as a rocky surface. The interplay between dust cover
528 and thermal behaviour has been already investigated on Mars by Crown & Ramsey (2017),
529 based on a THEMIS IR survey in Arsia Mons. These authors describe the difficulties in
530 discriminating the effect of mantling and albedo from the real thermal inertia, but they were
531 able to identify two types of lava flow: one group with relatively high albedo and large extent 23
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532 and a second one darker, smooth and smaller, associated with elongated channels and fissures,
533 very similar to the lava flow observed in Pyrrhae Chaos.
534
535 Despite the inferred younger age of the lava flow with respect to the surrounding basaltic
536 bedrock, we found that the lava flow predates the collapse since it is cut by a volcanic graben.
537 Thus, the lava flow could be interpreted as the last recognizable volcanic resurfacing event,
538 probably toward the end of the hypothesized multi-phase volcanic activity. The end of lava
539 supply could have been responsible for the collapse of the plumbing system generating the
540 volcanic grabens.
541 4.4 Non-disrupted units
542 The Light-toned Layered Unit and the Cap Unit were interpreted as sedimentary deposits first
543 of all by analogy with the nearby chaotic terrains, where deposits sharing the same
544 characteristics of those in Arsinoes Chaos were already interpreted as sedimentary units and
545 spectral analyses were already carried out (Dobrea et al., 2008; Glotch et al., 2005; Glotch &
546 Christensen, 2005; Glotch & Rogers, 2007; Lichtenberg et al., 2010; Sowe et al., 2012).
547 Furthermore, we interpreted the non-disrupted units as sedimentary for the following reasons:
548 Lack of volcano-tectonic features, high albedo, planar bedding, more prone to erosion than the
549 surrounding basalts, and mineralogical variation that could be associated with occurrence of
550 hydrated minerals
551
552 The reason why the NIR absorption features are not as prominent as in other CRISM data from
553 Aureum and Aram Chaos may be due to several factors: a lower abundance of hydrated
554 minerals, absence of areas bereft of hydrated minerals within the CRISM cube (thus the 24
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555 denominator cannot be ideal), lack of totally dust-free areas, features below the detection limit.
556 Especially for the Cap Unit, it was not possible to determine from a compositional point of
557 view the presence of the hydrated silicates detected in other similar sedimentary units because
558 even in the RGB with combined summary products, no significant variation was appreciable.
559 Nevertheless, the observed variation in the Light-toned Layered Unit could be consistent with
560 the interpretation of the layered deposits in the adjacent chaotic terrains, except for the high
561 concentration of hematite that was well described in Aram and Aureum Chaos but was not
562 found in Arsinoes Chaos. The hematite in Aram Chaos is considered by Glotch & Christensen
563 (2005) as a fundamental key for a lacustrine interpretation together with the bedding and the
564 closed geometry of the basin and the outflow channels. Despite the lack of hematite (missing
565 or simply below the detection limit) and outflow channels in Arsinoes Chaos, the hypothesis
566 of a lake or evaporitic basin as a depositional environment for the Light-toned Layered Unit
567 cannot be excluded either. We consider reasonable that a closed and deep basin such as
568 Arsinoes Chaos, that seems to possibly host hydrated minerals, might have been filled by
569 groundwater after the collapse.
570 It remains uncertain why in Pyrrhae Chaos the sequence ends with the Chaotic Terrain Unit.
571 The depth of the basin is approximately the same as Arsinoes Chaos (even deeper) and their
572 proximity would suggest that the same processes should have acted, but for some reason the
573 deposition of sedimentary units after the collapse did not happen in Pyrrhae Chaos (or the
574 deposits were completely eroded).
575 The exhumed light-toned deposit found in the north-eastern periphery of Arsinoes Chaos were
576 instead interpreted as related to hydrothermal activity that could have started as soon as the
577 volcanic activity responsible for the collapse has begun to stabilize. This hypothesis is
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578 supported by the evidence of smectite within the deposit. In this case, the hydrated minerals
579 are not in a closed basin, but associated with the volcano-tectonic structures that we interpreted
580 as directly related to volcanic collapses. Furthermore, the associations of basaltic minerals
581 (pyroxenes) and hydrated Fe-Mg phyllosilicates (smectite) can be explained by a process of
582 hydrothermal alteration, as summarized by Inoue (1995). According to that author, smectite
583 can be the result of hydrothermal alteration of andesitic to basaltic compositions under neutral
584 or alkaline conditions.
585 On Earth, this has been observed in several geological contexts, including stagnant
586 hydrothermal alteration of caldera deposits (Inoue et al., 1984). The extent of the hydrothermal
587 deposit is limited to 1.3 km and for scale reasons it was not possible to include it in the
588 geomorphic map. Nevertheless, in the surrounding area light-toned exhumed patches are
589 visible, suggesting an extent of the hydrothermal deposit of at least 10 km.
590
591 5 Conclusion
592 The morpho-stratigraphic mapping performed in Arsinoes Chaos highlighted the occurrence of
593 two major groups of geomorphic units: the basaltic Chaotic Terrain Unit, further subdivided
594 into three subunits (Fractured Plain, Knobby Terrain and High Thermal Inertia Chaotic Terrain)
595 represents the bedrock of the area and it is characterized by polygonal irregular mesas and
596 rounded knobs; the second group is composed by two non-disrupted units (Light-toned Layered
597 Unit and Cap Unit) that were deposited after the collapse of the Chaotic Terrain and lie
598 unconformably on top of the bedrock.
599 The Light-toned Layered Unit is characterized by a planar bedding and the scalloped surfaces
600 show high albedo; the Cap Unit seems instead to be a single thick layer (plateau-like) and lies
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601 unconformably on top of the Light-toned Layered Unit. In Pyrrhae Chaos the non-disrupted
602 units are missing.
603 The spectral analyses performed in Arsinoes Chaos could not entirely confirm the morpho-
604 stratigraphic evidences for the sedimentary deposits with the presence of hydrated phase
605 similar to the case of Aram and Aureum lying on the bedrock of the Chaotic Terrain Unit.
606 Nevertheless, a mineralogical variation is present and the existence of hydrated minerals below
607 CRISM detection limits cannot be ruled out.
608 The bedrock was confirmed to be basaltic in composition due to the occurrence of mafic
609 minerals, in particular low-Ca pyroxenes; the CRISM analyses did not reveal crucial
610 information on the Cap Unit and Light-toned Layered Unit, but we detected hydrated minerals
611 hosted by a small deposit located in proximity of the collapse-related structure in the
612 northeastern periphery of Arsinoes Chaos and we identified hydrated Fe-Mg phyllosilicates
613 (likely smectite). While the Light-toned Layered Unit may be explained by a
614 lacustrine/evaporitic environment that could have been established after the collapse and the
615 stabilization of the volcanic activity, the hydrated minerals occurring in the northeastern
616 periphery cannot be explained by such a hypothesis because they are not placed into a closed
617 basin, but they drape the volcano-tectonic structures interpreted as volcanic graben. For this
618 reason, the most likely hypothesis involves a hydrothermal system where hot water rises
619 through the fractures, deposits hydrated minerals and alters the pre-existing basaltic bedrock.
620 The structural evidences of volcano-tectonic activity support this interpretation, suggesting that
621 after the collapse the volcanic activity may have turned into a hydrothermal environment
622 warming up the groundwater that was infiltrating the fractures and finally reaching the surface
623 with processes of deposition and alteration. Given the lack of evidence for aqueous activity
624 pre- and syn-collapse, and the occurrence of widespread volcanic features, we infer the collapse 27
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625 to have been triggered primarily by volcano-tectonic processes, possibly interacting with
626 volatiles and/or permafrost. Further studies will reinforce or rule out this hypothesis.
627 6 Acknowledgments
628 We acknowledge support and funding from the European Union's Horizon 2020 research
629 and innovation programme under grant agreement Nº776276 (PLANMAP). We are grateful to
630 the reviewers and editors for their helpful comments. We also want to express our appreciation
631 for the insights and the fruitful discussions with Dr. Riccardo Pozzobon.
632 The processed CRISM cubes and the geopackage of the morpho-stratigraphic map, including
633 vectors and the raster of the basemap was stored in a repository following the FAIR principles
634 (Luzzi et al., 2020). The CTX mosaic is described by Dickson et al. (2018) and since it is a
635 beta version it does not have a DOI yet. A grid shapefile with the tiles and corresponding links
636 can be downloaded at http://murray-lab.caltech.edu/CTX/tiles/beta01/Murray-Lab_CTX-
637 Mosaic_beta01_QuadMap.zip. Alternatively the single CTX images can be downloaded at
638 https://ode.rsl.wustl.edu/mars/indexproductsearch.aspx after setting the adequate parameters
639 (MRO, CTX, Arsinoes Chaos, Pyrrhae Chaos). The HiRISE dataset is included in McEwen et
640 al. (2007). The HRSC dataset is included in Walter & van Gasselt (2014).
641
642 7 References
643
644 Alexis, J., Rodriguez, P., Kargel, J. S., Baker, V. R., Gulick, V. C., Berman, D. C., … Glines, N.
645 (2015). Martian outflow channels: How did their source aquifers form, and why did they drain
646 so rapidly? OPEN. Nature Publishing Group. https://doi.org/10.1038/srep13404
28
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
647 Andrews-Hanna, J. C., & Phillips, R. J. (2007). Hydrological modeling of outflow channels and
648 chaos regions on Mars. Journal of Geophysical Research: Planets, 112(E8).
649 Baker, V. R. (2001). Water and the Martian landscape. Nature, 412(6843), 228.
650 Bamberg, M., Jaumann, R., Asche, H., Kneissl, T., & Michael, G. G. (2014). Floor-fractured craters
651 on mars - Observations and origin. Planetary and Space Science, 98, 146–162.
652 https://doi.org/10.1016/j.pss.2013.09.017
653 Beyer, R. A., Alexandrov, O., & McMichael, S. (2018). The Ames Stereo Pipeline: NASA’s Open
654 Source Software for Deriving and Processing Terrain Data. Earth and Space Science, 5(9),
655 537–548. https://doi.org/10.1029/2018EA000409
656 Carr, M. H. (1979). Formation of Martian flood features by release of water from confined aquifers.
657 Journal of Geophysical Research, 84(B6), 2995. https://doi.org/10.1029/JB084iB06p02995
658 Catling, D. C., & Moore, J. M. (2003). The nature of coarse-grained crystalline hematite and its
659 implications for the early environment of Mars. Icarus, 165(2), 277–300.
660 https://doi.org/10.1016/S0019-1035(03)00173-8
661 Chapman, M. G., & Tanaka, K. L. (2002). Related magma--ice interactions: Possible origins of
662 chasmata, chaos, and surface materials in Xanthe, Margaritifer, and Meridiani Terrae, Mars.
663 Icarus, 155(2), 324–339.
664 Christensen, P. R., Engle, E., Anwar, S., Dickenshied, S., Noss, D., Gorelick, N., & Weiss-Malik,
665 M. (2009). JMARS - A Planetary GIS. American Geophysical Union, Fall Meeting 2009,
666 Abstract Id.IN22A-06. Retrieved from http://adsabs.harvard.edu/abs/2009AGUFMIN22A..06C
667 Christensen, P. R., Morris, R. V., Lane, M. D., Bandfield, J. L., & Malin, M. C. (2001). Global
668 mapping of Martian hematite mineral deposits: Remnants of water-driven processes on early
29
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
669 Mars. Journal of Geophysical Research: Planets, 106(E10), 23873–23885.
670 https://doi.org/10.1029/2000JE001415
671 Christensen, P R, Bandfield, J. L., Hamilton, V. E., Ruff, S. W., Kieffer, • H H, Titus, T. N., …
672 Greenfield, M. (2001). Mars Global Surveyor Thermal Emission Spectrometer experiment:
673 Investigation description and surface science results. In JOURNAL OF GEOPHYSICAL
674 RESEARCH (Vol. 106). https://doi.org/10.1029/2000JE001370
675 Christensen, Philip R., Bandfield, J. L., Smith, M. D., Hamilton, V. E., & Clark, R. N. (2000).
676 Identification of a basaltic component on the Martian surface from Thermal Emission
677 Spectrometer data. Journal of Geophysical Research: Planets, 105(E4), 9609–9621.
678 https://doi.org/10.1029/[email protected]/(ISSN)2169-9100.MGSTES1
679 Christensen, Philip R., Jakosky, B. M., Kieffer, H. H., Malin, M. C., McSween, Jr., H. Y., Nealson,
680 K., … Ravine, M. (2004). The Thermal Emission Imaging System (THEMIS) for the Mars
681 2001 Odyssey Mission. Space Science Reviews, 110(1/2), 85–130.
682 https://doi.org/10.1023/B:SPAC.0000021008.16305.94
683 Clark, R. N., King, T. V. V., Klejwa, M., Swayze, G. A., & Vergo, N. (1990). High spectral
684 resolution reflectance spectroscopy of minerals. Journal of Geophysical Research, 95(B8).
685 https://doi.org/10.1029/jb095ib08p12653
686 Cloutis, E. A., Gaffey, M. J., Jackowski, T. L., & Reed, K. L. (1986). Calibrations of phase
687 abundance, composition, and particle size distribution for olivine-orthopyroxene mixtures from
688 reflectance spectra. Journal of Geophysical Research, 91(B11), 11641.
689 https://doi.org/10.1029/jb091ib11p11641
690 Crown, D. A., & Ramsey, M. S. (2017). Morphologic and thermophysical characteristics of lava
691 flows southwest of Arsia Mons, Mars. Journal of Volcanology and Geothermal Research, 342, 30
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
692 13–28. https://doi.org/10.1016/J.JVOLGEORES.2016.07.008
693 Dickson, J., Kerber, L., Fassett, C., & Ehlmann, B. (2018). A Global, Blended CTX Mosaic of Mars
694 With Vectorized Seam Mapping: A New Mosaicking Pipeline Using Principles of Non-
695 Destructive Image Editing. Lunar and Planetary Science Conference.
696 Dobrea, E. Z. N., Poulet, F., & Malin, M. C. (2008). Correlations between hematite and sulfates in
697 the chaotic terrain east of Valles Marineris. Icarus, 193(2), 516–534.
698 Dombard, A. J., & Gillis, J. J. (2001). Testing the viability of topographic relaxation as a
699 mechanism for the formation of lunar floor-fractured craters. Journal of Geophysical
700 Research: Planets, 106(E11), 27901–27909. https://doi.org/10.1029/2000JE001388
701 Fergason, R. L., Christensen, P. R., & Kieffer, H. H. (2006). High‐resolution thermal inertia
702 derived from the Thermal Emission Imaging System (THEMIS): Thermal model and
703 applications. https://doi.org/10.1029/2006JE002735
704 Ferrill, D. A., Wyrick, D. Y., & Smart, K. J. (2011). Coseismic, dilational-fault and extension-
705 fracture related pit chain formation in Iceland: Analog for pit chains on Mars. Lithosphere,
706 3(2), 133–142. https://doi.org/10.1130/L123.1
707 Flahaut, J., Mustard, J. F., Quantin, C., Clenet, H., Allemand, P., & Thomas, P. (2011). Dikes of
708 distinct composition intruded into Noachian-aged crust exposed in the walls of Valles
709 Marineris. Geophysical Research Letters, 38(15). https://doi.org/10.1029/2011GL048109
710 G Walter sebastianwalter, S. H., & van Gasselt, S. (n.d.). HRSC Data Dissemination-Dynamic
711 Queries and Data Interoperability. Retrieved from http://maps.planet.fu-berlin.de
712 Gaddis, L., Anderson, J., Becker, K., Becker, T., Cook, D., Edwards, K., … Robinson, M. (1997).
713 An Overview of the Integrated Software for Imaging Spectrometers (ISIS). Lunar and
31
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
714 Planetary Science Conference, 387. Retrieved from
715 https://ui.adsabs.harvard.edu/abs/1997LPI....28..387G/abstract
716 Gendrin, A., Mangold, N., Bibring, J.-P., Langevin, Y., Gondet, B., Poulet, F., … LeMouélic, S.
717 (2005). Sulfates in Martian layered terrains: the OMEGA/Mars Express view. Science (New
718 York, N.Y.), 307(5715), 1587–1591. https://doi.org/10.1126/science.1109087
719 Glotch, T D, Rogers, D., & Christensen, P. R. (2005). A newly discovered hematite-rich unit in
720 Aureum Chaos: Comparison of hematite and associated units with those in Aram Chaos. 36th
721 Annual Lunar and Planetary Science Conference, 36.
722 Glotch, Timothy D, & Christensen, P. R. (2005). Geologic and mineralogic mapping of Aram
723 Chaos: Evidence for a water-rich history. Journal of Geophysical Research: Planets, 110(E9).
724 Glotch, Timothy D, & Rogers, A. D. (2007). Evidence for aqueous deposition of hematite‐ and
725 sulfate‐rich light‐toned layered deposits in Aureum and Iani Chaos, Mars.
726 https://doi.org/10.1029/2006JE002863
727 Hall, J. L., Solomon, S. C., & Head, J. W. (1981). Lunar floor-fractured craters: Evidence for
728 viscous relaxation of crater topography. Journal of Geophysical Research: Solid Earth,
729 86(B10), 9537–9552. https://doi.org/10.1029/jb086ib10p09537
730 Harrison, K. P., & Grimm, R. E. (2008). Multiple flooding events in Martian outflow channels.
731 Journal of Geophysical Research, 113(E2), E02002. https://doi.org/10.1029/2007JE002951
732 Harrison, K. P., & Grimm, R. E. (2009). Regionally compartmented groundwater flow on Mars.
733 Journal of Geophysical Research: Planets, 114(E4).
734 Head, J. W., & Wilson, L. (2002). Mars: a review and synthesis of general environments and
735 geological settings of magma-H 2 O interactions. Geological Society, London, Special
32
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
736 Publications, 202(1), 27–57. https://doi.org/10.1144/GSL.SP.2002.202.01.03
737 Head, J. W., & Wilson, L. (2007). Heat transfer in volcano–ice interactions on Mars: synthesis of
738 environments and implications for processes and landforms. Annals of Glaciology, 45, 1–13.
739 https://doi.org/10.3189/172756407782282570
740 Hoffman, N. (2000). White Mars: A New Model for Mars’ Surface and Atmosphere Based on CO2.
741 Icarus, 146(2), 326–342. https://doi.org/10.1006/ICAR.2000.6398
742 Hoke, M. R. T., Hynek, B. M., & Tucker, G. E. (2011). Formation timescales of large Martian
743 valley networks. Earth and Planetary Science Letters, 312(1–2), 1–12.
744 https://doi.org/10.1016/j.epsl.2011.09.053
745 Inoue, A.; Utada, M.; Nagata, U.; Watanabe, T. (1984). Conversion of trioctahedral smectite to
746 interstratified Chlorite/Smectite in Pliocene acidic pyroclastic sediments of the Ohyu district,
747 Akita Prefecture, Japan. Clay Science, 6(3), 103–116.
748 https://doi.org/10.11362/jcssjclayscience1960.6.103
749 Inoue, A. (1995). Formation of Clay Minerals in Hydrothermal Environments. In Origin and
750 Mineralogy of Clays (pp. 268–329). https://doi.org/10.1007/978-3-662-12648-6_7
751 Irwin, R. P., Tooth, S., Craddock, R. A., Howard, A. D., & De Latour, A. B. (2014). Origin and
752 development of theater-headed valleys in the atacama desert, northern chile: Morphological
753 analogs to martian valley networks. Icarus, 243, 296–310.
754 https://doi.org/10.1016/j.icarus.2014.08.012
755 Jaeger, W. L., Keszthelyi, L. P., Skinner, J. A., Milazzo, M. P., McEwen, A. S., Titus, T. N., …
756 Kirk, R. L. (2010). Emplacement of the youngest flood lava on Mars: A short, turbulent story.
757 Icarus, 205(1), 230–243. https://doi.org/10.1016/j.icarus.2009.09.011
33
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
758 Jakosky, B. M., Mellon, M. T., Kieffer, H. H., Christensen, P. R., Varnes, E. S., & Lee, S. W.
759 (2000). The thermal inertia of Mars from the Mars Global Surveyor Thermal Emission
760 Spectrometer. Journal of Geophysical Research: Planets, 105(E4), 9643–9652.
761 https://doi.org/10.1029/1999JE001088
762 Jozwiak, L. M., Head, J. W., & Wilson, L. (2015). Lunar floor-fractured craters as magmatic
763 intrusions: Geometry, modes of emplacement, associated tectonic and volcanic features, and
764 implications for gravity anomalies. Icarus, 248, 424–447.
765 https://doi.org/10.1016/j.icarus.2014.10.052
766 Jozwiak, L. M., Head, J. W., Zuber, M. T., Smith, D. E., & Neumann, G. A. (2012). Lunar floor-
767 fractured craters: Classification, distribution, origin and implications for magmatism and
768 shallow crustal structure. Journal of Geophysical Research E: Planets, 117(11).
769 https://doi.org/10.1029/2012JE004134
770 Kargel, J. S., Furfaro, R., Prieto-Ballesteros, O., Rodriguez, J. A. P., Montgomery, D. R., Gillespie,
771 A. R., … Wood, S. E. (2007). Martian hydrogeology sustained by thermally insulating gas and
772 salt hydrates. Geology, 35(11), 975–978.
773 King, T. V. V., & Ridley, W. I. (1987). Relation of the spectroscopic reflectance of olivine to
774 mineral chemistry and some remote sensing implications. Journal of Geophysical Research:
775 Solid Earth, 92(B11), 11457–11469. https://doi.org/10.1029/JB092iB11p11457
776 Kneissl, T., Van Gasselt, S., Wendt, L., Gross, C., & Neukum, G. (2011). Layering and degradation
777 of the Rupes Tenuis unit, Mars--a structural analysis south of Chasma Boreale. Geological
778 Society, London, Special Publications, 356(1), 257–279.
779 Koeppen, W. C., & Hamilton, V. E. (2008). Global distribution, composition, and abundance of
780 olivine on the surface of Mars from thermal infrared data. Journal of Geophysical Research E: 34
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
781 Planets, 113(5). https://doi.org/10.1029/2007JE002984
782 Lamb, M. P., Howard, A. D., Johnson, J., Whipple, K. X., Dietrich, W. E., & Perron, J. T. (2006).
783 Can springs cut canyons into rock? Journal of Geophysical Research, 111(E7), E07002.
784 https://doi.org/10.1029/2005JE002663
785 Le Deit, L., Le Mouélic, S., Bourgeois, O., Combe, J.-P., Mège, D., Sotin, C., … Bibring, J.-P.
786 (2008). Ferric oxides in East Candor Chasma, Valles Marineris (Mars) inferred from analysis
787 of OMEGA/Mars Express data: Identification and geological interpretation. Journal of
788 Geophysical Research, 113(E7), E07001. https://doi.org/10.1029/2007JE002950
789 Leask, H. J., Wilson, L., & Mitchell, K. L. (2006). Formation of Aromatum Chaos, Mars:
790 Morphological development as a result of volcano-ice interactions. Journal of Geophysical
791 Research: Planets, 111(E8).
792 Leask, H. J., Wilson, L., & Mitchell, K. L. (2007). Formation of Mangala Valles outflow channel,
793 Mars: Morphological development and water discharge and duration estimates. Journal of
794 Geophysical Research: Planets, 112(E8). https://doi.org/10.1029/2006JE002851
795 Leone, G. (2014). A network of lava tubes as the origin of Labyrinthus Noctis and Valles Marineris
796 on Mars. Journal of Volcanology and Geothermal Research, 277, 1–8.
797 Leverington, D. W. (2011, September 15). A volcanic origin for the outflow channels of Mars: Key
798 evidence and major implications. Geomorphology, Vol. 132, pp. 51–75.
799 https://doi.org/10.1016/j.geomorph.2011.05.022
800 Lichtenberg, K. A., Arvidson, R. E., Morris, R. V., Murchie, S. L., Bishop, J. L., Fernandez
801 Remolar, D., … Roach, L. H. (2010). Stratigraphy of hydrated sulfates in the sedimentary
802 deposits of Aram Chaos, Mars. Journal of Geophysical Research, 115(E6), E00D17.
35
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
803 https://doi.org/10.1029/2009JE003353
804 Liu, Y., Arvidson, R. E., Wolff, M. J., Mellon, M. T., Catalano, J. G., Wang, A., & Bishop, J. L.
805 (2012). Lambert albedo retrieval and analyses over Aram Chaos from OMEGA hyperspectral
806 imaging data. J. Geophys. Res, 117, 0–11. https://doi.org/10.1029/2012JE004056
807 Luzzi, E. R. A. P. C. C. A. F. (2020). Geological Map of Arsinoes and Pyrrhae Chaos, Mars.
808 Retrieved June 10, 2020, from https://data.4tu.nl/repository/uuid:c4b7c436-5af3-449f-9757-
809 908ca5a73258/object/citation
810 Malin, M. C., Bell, J. F., Cantor, B. A., Caplinger, M. A., Calvin, W. M., Clancy, R. T., … Wolff,
811 M. J. (2007). Context Camera Investigation on board the Mars Reconnaissance Orbiter.
812 Journal of Geophysical Research: Planets, 112(E5).
813 Manker, J. P., & Johnson, A. P. (1982). Simulation of Martian chaotic terrain and outflow channels.
814 Icarus, 51(1), 121–132. https://doi.org/10.1016/0019-1035(82)90032-X
815 Massé, M., Mouélic, S. Le, Bourgeois, O., Combe, J.-P., Deit, L. Le, Sotin, C., … Langevin, Y.
816 (2008). Mineralogical composition, structure, morphology, and geological history of Aram
817 Chaos crater fill on Mars derived from OMEGA Mars Express data. J. Geophys. Res, 113,
818 12006. https://doi.org/10.1029/2008JE003131
819 McEwen, A. S., Eliason, E. M., Bergstrom, J. W., Bridges, N. T., Hansen, C. J., Delamere, W. A.,
820 … Weitz, C. M. (2007). MRO’s High Resolution Imaging Science Experiment (HiRISE).
821 Journal of Geophysical Research: Planets, 112(E5).
822 Mège, D., & Gurgurewicz, J. (2016). On Mars, Location and Orientation of Dykes Exposed along
823 the Valles Marineris Walls Reveal Expected and Unexpected Stress Fields. Acta Geologica
824 Sinica - English Edition, 90(s1), 177–179. https://doi.org/10.1111/1755-6724.12959
36
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
825 Mège, D., & Masson, P. (1996). Amounts of crustal stretching in Valles Marineris, Mars. In Plmet.
826 Space Sci (Vol. 44).
827 Mellon, M. T., Jakosky, B. M., Kieffer, H. H., & Christensen, P. R. (2000). High-Resolution
828 Thermal Inertia Mapping from the Mars Global Surveyor Thermal Emission Spectrometer.
829 Icarus, 148, 437–455. https://doi.org/10.1006/icar.2000.6503
830 Meresse, S., Costard, F., Mangold, N., Masson, P., Neukum, G., & others. (2008). Formation and
831 evolution of the chaotic terrains by subsidence and magmatism: Hydraotes Chaos, Mars.
832 Icarus, 194(2), 487–500.
833 Montési, L. G. J. (2001). Concentric dikes on the flanks of Pavonis Mons: Implications for the
834 evolution of martian shield volcanoes and mantle plumes. Special Paper of the Geological
835 Society of America, 352, 165–181. https://doi.org/10.1130/0-8137-2352-3.165
836 Moratto, Z. M., Broxton, M. J., Beyer, R. A., Lundy, M., & Husmann, K. (2010). Ames Stereo
837 Pipeline, NASA’s open source automated stereogrammetry software. Lunar and Planetary
838 Science Conference, 41, 2364.
839 Mouginis-Mark, P. J., & Christensen, P. R. (2005). New observations of volcanic features on Mars
840 from the THEMIS instrument. Journal of Geophysical Research: Planets, 110(E8).
841 Murchie, S., Arvidson, R., Bedini, P., Beisser, K., Bibring, J.-P., Bishop, J., … Wolff, M. (2007).
842 Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on Mars Reconnaissance
843 Orbiter (MRO). Journal of Geophysical Research, 112(E5), E05S03.
844 https://doi.org/10.1029/2006JE002682
845 Okubo, C. H., & Martel, S. J. (n.d.). Pit crater formation on Kilauea volcano, Hawaii. In Journal of
846 Volcanology and Geothermal Research (Vol. 86).
37
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
847 Okubo, C. H., & Schultz, R. A. (2005). Evidence of normal faulting and dike intrusion at Valles
848 Marineris from pit crater topography.
849 Ormö, J., Komatsu, G., Chan, M. A., Beitler, B., & Parry, W. T. (2004). Geological features
850 indicative of processes related to the hematite formation in Meridiani Planum and Aram
851 Chaos, Mars: a comparison with diagenetic hematite deposits in southern Utah, USA. Icarus,
852 171, 295–316. https://doi.org/10.1016/j.icarus.2004.06.001
853 Pozzobon, R., Mazzarini, F., Massironi, M., & Marinangeli, L. (2015). Self-similar clustering
854 distribution of structural features on Ascraeus Mons (Mars): implications for magma chamber
855 depth. Geological Society, London, Special Publications, 401(1), 203–218.
856 Roda, M., Kleinhans, M. G., Zegers, T. E., & Oosthoek, J. H. P. (2014). Catastrophic ice lake
857 collapse in Aram Chaos, Mars. Icarus, 236, 104–121.
858 Rodriguez, J. A. P., Sasaki, S., Kuzmin, R. O., Dohm, J. M., Tanaka, K. L., Miyamoto, H., …
859 Ferris, J. C. (2005). Outflow channel sources, reactivation, and chaos formation, Xanthe Terra,
860 Mars. Icarus, 175(1), 36–57.
861 Schmidt, G., Fueten, F., Stesky, R., Flahaut, J., & Hauber, E. (2018). Geology of Hebes Chasma,
862 Mars: 1. Structure, Stratigraphy, and Mineralogy of the Interior Layered Deposits. Journal of
863 Geophysical Research: Planets, 123(11), 2893–2919. https://doi.org/10.1029/2018JE005658
864 Schultz, P. H., Schultz, R. A., & Rogers, J. (1982). The structure and evolution of ancient impact
865 basins on Mars. Journal of Geophysical Research: Solid Earth, 87(B12), 9803–9820.
866 Scott, E. D., & Wilson, L. (1999). Evidence for a sill emplacement event on the upper flanks of the
867 Ascraeus Mons shield volcano, Mars. Journal of Geophysical Research: Planets, 104(E11),
868 27079–27089. https://doi.org/10.1029/1999JE001049
38
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
869 Scott, E. D., & Wilson, L. (2002). Plinian eruptions and passive collapse events as mechanisms of
870 formation for Martian pit chain craters. Journal of Geophysical Research E: Planets, 107(4),
871 4–1. https://doi.org/10.1029/2000je001432
872 Sefton-Nash, E., Catling, D. C., Wood, S. E., Grindrod, P. M., & Teanby, N. A. (2012).
873 Topographic, spectral and thermal inertia analysis of interior layered deposits in Iani Chaos,
874 Mars. Icarus, 221(1), 20–42. https://doi.org/10.1016/j.icarus.2012.06.036
875 Sowe, M, Wendt, L., McGuire, P. C., & Neukum, G. (2012). Hydrated minerals in the deposits of
876 Aureum Chaos. Icarus, 218(1), 406–419.
877 Sowe, Mariam, Stesky, R., Fueten, F., Sowe, M., Hauber, E., Jaumann, R., … Neukum, G. (2007).
878 LAYERED DEPOSITS OF IANI CHAOS ON MARS Interior Layered Deposits in the
879 Eastern Valles Marineris and Chaotic Terrains on Mars. Lunar Planet. Sci., XXXVIII.
880 https://doi.org/10.1029/2004JE002389
881 Survey, U. S. G. (2006). FGDC Digital Cartographic Standard for Geologic Map Symbolization
882 (PostScript Implementation) Prepared in cooperation with the Geologic Data Subcommittee of
883 the\rFederal Geographic Data Committee. U.S. Geological Survey Techniques and Methods
884 11-A2.
885 Tanaka, K. L. & G. M. P. (1989). Martian tension fractures and the formation of grabens and
886 collapse features at Valles Marineris. Lunar and Planetary Science Conference, 19th, Houston,
887 TX, Mar. 14-18, 1988, Proceedings (A89-36486 15-91), 383–396. Retrieved from
888 http://articles.adsabs.harvard.edu/cgi-bin/nph-
889 iarticle_query?bibcode=1989LPSC...19..383T&db_key=AST&page_ind=10&plate_select=N
890 O&data_type=GIF&type=SCREEN_GIF&classic=YES
891 Tanaka, K. L., Robbins, S. J., Fortezzo, C. M., Skinner Jr, J. A., & Hare, T. M. (2014). The digital 39
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
892 global geologic map of Mars: Chronostratigraphic ages, topographic and crater morphologic
893 characteristics, and updated resurfacing history. Planetary and Space Science, 95, 11–24.
894 Tange, O. (2011). GNU Parallel: The Command-Line Power Tool | USENIX. The USENIX
895 Magazine, 3(1), 42–47. Retrieved from https://www.usenix.org/publications/login/february-
896 2011-volume-36-number-1/gnu-parallel-command-line-power-tool
897 Thorey, C., & Michaut, C. (2014). A model for the dynamics of crater-centered intrusion:
898 Application to lunar floor-fractured craters. Journal of Geophysical Research: Planets, 119(1),
899 286–312. https://doi.org/10.1002/2013JE004467
900 Viviano-Beck, C. E., Seelos, F. P., Murchie, S. L., Kahn, E. G., Seelos, K. D., Taylor, H. W., …
901 others. (2014). Revised CRISM spectral parameters and summary products based on the
902 currently detected mineral diversity on Mars. Journal of Geophysical Research: Planets,
903 119(6), 1403–1431.
904 Wichman, R. W., & Schultz, P. H. (1996). Crater-centered laccoliths on the Moon: Modeling
905 intrusion depth and magmatic pressure at the crater Taruntius. Icarus, 122(1), 193–199.
906 https://doi.org/10.1006/icar.1996.0118
907 Wilson, L., & Head III, J. W. (2002). Tharsis-radial graben systems as the surface manifestation of
908 plume-related dike intrusion complexes: Models and implications. Journal of Geophysical
909 Research: Planets, 107(E8), 1.
910 Wyrick, D. (2004). Distribution, morphology, and origins of Martian pit crater chains. Journal of
911 Geophysical Research, 109(E6), E06005. https://doi.org/10.1029/2004JE002240
912 Zegers, T. E., Oosthoek, J. H. P., Rossi, A. P., Blom, J. K., & Schumacher, S. (2010). Melt and
913 collapse of buried water ice: An alternative hypothesis for the formation of chaotic terrains on
40
SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
914 Mars. Earth and Planetary Science Letters, 297(3–4), 496–504.
915
916
917 8 Tables and captions
918 Fig. 1: Location of Arsinoes and Pyrrhae Chaos on a MOLA Global Color Shaded Relief.
919
920 Figure 2: Elevation contours in Pyrrhae Chaos showing the mesas of the Fractured plains at higher
921 elevations compared the mounds of the Knobby Terrain. CTX mosaic; HRSC DEM from orbit
922 h1958_0000.
923
924 Figure 3: A) Overview map of the area where the contact between the two non-disrupted units occurs.
925 The black square bounds the area depicted in D. The section X-X’ refers to the topographic profile in
926 C. B) 3D view of the stratigraphic contact between the Light-toned Layered Unit and the Cap Unit,
927 separated by the unconformity (in blue). C) Topographic profile showing the plateau-like morphology
928 of the Cap Unit and the different attitude of the units. D) Some of the attitudes measured with
929 LayerTools (light blue: Light-toned Layered Unit; black: Cap Unit). CTX stereo pair:
930 B05_011700_1720_XI_08S028W and B06_012056_1721_XI_07S028W.
931
932 Figure 4: The light-toned layered deposits overlying the Knobby Terrain and outflanking the mounds
933 (black arrows). The superposition relationships suggest a younger age of the layered deposits
934 relatively to the knobs. Note also the tendency to assume a lobate morphology. CTX:
935 F10_039563_1729_XN_07S027W.
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936
937 Figure 5: Faults and the elongated graben-like depressions mapped in red in Arsinoes Chaos (A) and
938 Pyrrhae Chaos (B). The locations of Fig. 5 B-C-D and Fig. 9 are indicated in Fig. 5A. C) The
939 elongated graben-like depressions occur close to the faults affecting the Fractured Plains and it is
940 difficult to distinguish their separation. D) The elongated graben-like depressions are also coalescent
941 with pit chain (black arrows) and are often associated with heavily pitted areas (pink area). CTX
942 mosaic.
943
944 Figure 6: Two wrinkle ridges are indicated by the black arrows. CTX mosaic.
945
946 Figure 7: Topographic profiles of the northeastern (top) and southeastern (bottom) craters. The
947 images for context are from the CTX mosaic, the DEM used for the profiles is a MOLA-HRSC
948 blended DEM.
949
950 Fig. 8: Contact between the younger lava flow and the underlying basalts, both cut by collapse-
951 related faults. HiRISE image ESP_037123_1690 (black & white in A and C, false color in B.
952
953 Figure 9: A) 3D view of the false colour HRSC image h1947_0000, showing a difference in tone
954 between the Fractured Plains materials and the lava flow indicated by the white arrow. The black
955 arrow points to a y-shaped conjunction within an elongated graben-like depression in the south-
956 eastern Pyrrhae Chaos. B) The same features displayed in A are now showed in another perspective
42
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957 and in THEMIS-Derived Global Thermal Inertia Mosaic. Note that the lava flow occurring in Pyrrhae
958 Chaos (white arrow) has a higher thermal inertia than the surrounding materials.
959
960 Figure 10: A) Location of the CRISM cube frt00008233_07 superposed on the geomorphic map
961 where the different units are shown. The complete geomorphic map with legend is presented in Fig.
962 15. B) True colors image (R=R600; G=R530; B=R440). C) RGB composite with summary
963 parameters of the CRISM TRDR frt00008233 scene. CHL: Chlorides are in blue, yellow/green are
964 indicative of hydrated minerals (R= ISLOPE; G= BD3000; B=IRR2).
965
966 Figure 11: Location of the peripheral CRISM cubes frt00023790_07 and frt000196b0_07 on the CTX
967 mosaic.
968
969 Figure 12: A) Infrared false color scene showing a mineralogical variation between the exhumed
970 deposits and the surrounding bedrock. B) RGB composite with summary products for phyllosilicates
971 with Fe and Mg (PFM: cyan colors, coinciding with the albedo variation, indicate Fe/Mg smectites,
972 R=BD2355; G=D2300; B=BD2290). C) The band depth at 2.3 μm is significant in correspondence
973 of the hydrated minerals. On the right, the ratioed spectrum is compared to resampled CRISM spectra
974 of Smectite 397S170, Illite LAIL01, and Hisingerite LAHI01 from the spectral library. The black
975 lines highlight the absorptions of the ratioed spectrum at 1.42, 1.92 and 2.28 μm. CRISM cube
976 frt000196b0_07.
977
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978 Figure 13: A) LCPINDEX shows the abundances of pyroxenes on the bedrock. B) Interpretation of
979 the mineralogical variation: in the central region the hydrated minerals are interpreted as Fe-Mg
980 phyllosilicates (smectite), while the bedrock rich in pyroxenes is indicative of basaltic compositions.
981 CRISM cube frt000196b0_07.
982
983 Figure 14: A) False color scene and location of s1 and s2; B) the ratioed spectrum is compared to the
984 CRISM resampled spectra for clinopyroxene C1PP4 and orthopyroxene CBSB51. Both the
985 absorption at 1 μm and the broad absorption at 2 μm are present and consistent with a mafic
986 composition.
987 CRISM cube frt00023790_07.
988
989 Figure 15: Morpho-stratigraphic map of Arsinoes and Pyrrhae Chaos.
990
991
992
993 Table 1: List of images and data cubes used in this work.
Dataset Image ID Stereo pair
HiRISE ESP_016658_1735 -
HiRISE ESP_020060_1715 -
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SUBMITTED TO JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS
994 HiRISE ESP_026996_1725 -
995 HiRISE ESP_027629_1730 -
996 HiRISE ESP_034499_1710 - 997
HiRISE ESP_035356_1715 - 998
999 HiRISE ESP_036622_1720 - 1000
HiRISE ESP_037545_1730 - 1001 1002 HiRISE ESP_039352_1730 - 1003
HiRISE ESP_053883_1730 -
HiRISE PSP_002180_1720 -
HiRISE ESP_037123_1690 -
CTX B05_011700_1720_XI_08S028W B06_012056_1721_XI_07S028W
CTX F10_039563_1729_XN_07S027W P04_002747_1736_XN_06S027W
CRISM frt00008233_07 -
CRISM frt00023790_07 -
CRISM frt000196b0_07 -
HRSC h1947_0000 -
HRSC h1958_0000
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